Re: Apple Graphics Tablet info?

[mjmahon@aol.com (MJMahon)]

I just found the manual, so here's a correction to my post (!):

>Frank McKenney wrote:
>
>> I don't really need the adapter...  just the pinouts and what they
>> represent.
>
>I don't think that will help as much as you think (see below).
>
>> I'm hoping to connect this tablet to either a Macintosh or an Intel PC by
>> building an interface using a PIC microprocessor ("Hi!  You thing I'm a
>> mouse...  but I'm really not!").  Trouble is, I have no idea what the
>> lines from the board - and the stylus - represent.  Makes converting it a
>> bit tricky (;-).
>> 
>> I'm guessing that the tablet senses position via resistance, and, since
>> there's no power connections or batter holder, that I need to supply the
>> power myself.  But...  what voltage, and what resistance values represent
>> what position?
>
>Ah, but it doesn't use resistance.  It is an RF device which operates
>at about 20 MHz if I remember correctly.  The pen is an inductive pickup
>which detects the RF pulse as it propagates through a tapped delay line
>for each axis.  The board contains the pulsed exciter for the delay lines
>and the RF "sense amplifier" for the pen, plus a high frequency counter
>to capture the time delay, and thus the position, of the pen.
>
>Driving this tablet without the original board is not for the faint
>of heart!   ;-(

Yes, but...   From the schematic, it appears to be just a current wavefront
that's sent down the delay lines.  There are separate I/O addresses to
fire the X- and Y-axis SCR's, which start a nice sharp edge propagating
down each axis in its turn.  A 12-bit counter is reset at the same time,
and counts up at a multiple of 7 MHz between 38 MHz and 54 MHz--
so either 42 MHz or 49 MHz.  When the inductive pen senses the
current edge, it stops the counter.  The Apple can then read the
counter value at other I/O addresses.

All in all, probably the fastest circuit ever to go into a //e!

>> Anyone have any ideas who might still have documentation on this device?
>> I checked Apple's 'web site, but no luck.
>
>There was a little (~25 page) manual for the tablet which was widely
>available as a separate manual up to about 1986 or so.  I have one,
>and it contains the schematic for the adapter.  Unfortunately, I haven't
>seen mine since 1990 and I haven't seen them for sale for even longer.
>
>If you're still in the mood, I can try to find my copy...

I did, and its 123 pages--a little thicker than I remembered!  ;-)


-michael

 Email:  mjmahon@aol.com
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